Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. Typically, one or more reference or pinned layers has a fixed magnetization, while a free layer has a magnetization that can be changed. Binary coded information, “1” or “0”, typically corresponds to the magnetizations of the free and reference layers being parallel or anti-parallel, respectively. Information is written by setting the magnetization direction(s) of the free layer(s) in a specified cell. This is typically accomplished either using a magnetic field generated by supplying a current to a write line disposed in a cross stripe shape, or directly using a current induced switching scheme utilizing the spin transfer effect.
FIGS. 1 and 2 depict portions of conventional MRAMs 10 and 30, respectively. Referring to FIG. 1, the conventional magnetic memory 10 includes magnetic storage elements, such as the magnetic storage element 20. The magnetic storage elements 20 are switched using an external magnetic field (field-based switching). The conventional MRAM 10 also includes a write word line 12, interlayer dielectric layers 14, metal 16, bypass 18, via 24, and bit line 26. Typically, the bit lines 26 and write word lines 12 run in perpendicular directions, with the magnetic storage elements 20 residing at the crossings. The magnetic storage element 20 is coupled to the underlying metal plug 16 through the bypass 18 and is insulated from the write word line 12. The magnetic storage element 20 is also coupled to the bit line 26 above through the via 24.
In the conventional MRAM 10, a magnetic field is used to switch the state of the magnetic storage element 20. In particular, currents may be driven through the bit line 26 and the write word line 12. However, one of ordinary skill in the art will recognize that the currents required to write to the magnetic storage element 20 do not scale down as the size of the magnetic cell 11 decreases. Because of this, a relatively high current may be needed to generate a magnetic field sufficient to switch the state of the magnetic storage element 20. Consequently, power consumption of the MRAM 10 increases and more reliability issues may be encountered due to the higher current carried by the bit line 26 and word line 12. The conventional field-based MRAM 10 may thus be unsuitable for use at higher densities.
In contrast, the conventional MRAM 30 of FIG. 2 may be written by driving a current through the magnetic storage element 38 (current-based switching). Generally, the MRAM 30 is written using the spin transfer effect. The conventional MRAM 30 thus includes magnetic storage elements, such as the magnetic storage element 38, that utilize current-based switching. For example, the magnetic storage element 38 might include a magnetic tunneling junction. In addition, the lateral dimensions of the magnetic storage element 38 may be small, for example in the range of a few hundred nanometers or less, in order to facilitate current-based switching through the spin transfer effect. The conventional MRAM 30 also includes a metal plug 32, interlayer dielectric layers 34 and 36, via 40, bit line 42, and a word line (not shown). Typically, the bit lines 42 and word lines run in perpendicular directions, with the magnetic storage elements 38 residing at the crossings. The magnetic storage element 38 is coupled to the underlying selection transistor (not shown) via a metal plug 32 and to the bit line 42 above through the via 40. Typically, a memory cell 11 or 31 includes the magnetic storage element 20 or 38, respectively, as well as a selection transistor (not shown). In addition, although depicted as being connected to the bit line 42 through the via 40, in some conventional magnetic memories, the magnetic storage element 38 may be directly connected to the bit line 42.
In the conventional MRAM 30, the current driven through the magnetic storage element 38 induces switching of the magnetization of the free layers of the magnetic storage elements 38. This current required to switch the free, or recording, layers (switching current) decreases as the MRAM 30 density grows, scaling down in a manner comparable to the semiconductor or CMOS technology evolution. The current density required to switch the magnetic element (switching current density), Jc, achievable is on the order of 106 A/cm2. Further, isolation transistors (not shown) used in conventional MRAM 30 have dimension that are proportional to the saturation current and thus scale with the switching current. This low switching current density for the MRAM 30 allows spin transfer switching to be useful in high density MRAM. In particular, the low switching current density may allow for lower power consumption and smaller isolation transistors than the conventional MRAM 10. Currently, the switching current corresponding to the low switching current density may be as small as 0.1 mA. This current is significantly smaller than the switching current used in generating the magnetic field for the conventional field-switched MRAM 10. Power consumption may thus be significantly reduced. Further, because smaller isolation transistors may be used, the size of the magnetic storage cell 31 may be reduced.
Although the lower switching current density is desirable, one of ordinary skill in the art will recognize that there are still issues with using the MRAM 30 at higher densities. In particular, size related issues and the initial conditions of the magnetic storage element 38 may adversely affect performance of the MRAM 30. For example, changes in initial magnetization or magnetic distribution of the free layer may radically affect the current-based switching of the free layer.
The affect of the initial conditions may be understood using current spin transfer models. According to a prevalent spin transfer model, at nanosecond switching regime the switching time, τ, for the magnetic cell 31 may be expressed as:
                    τ        =                              2                          α              ⁢                                                          ⁢              γ              ⁢                                                          ⁢                              M                s                                              ⁢                                    I                              c                ⁢                                                                  ⁢                0                                                                    I                pulse                            -                              I                                  c                  ⁢                                                                          ⁢                  0                                                              ⁢          ln          ⁢                                          ⁢                      π                          2              ⁢                                                          ⁢                              θ                0                                                                        (        1        )            
where α, γ and Ms, are the phenomenological Gilbert damping, the gyromagnetic ratio and saturation magnetization of the free layer, respectively. In spin transfer switching, a precession mode of initial angle θ0 id pumped until the amplitude reaches π/2 with an overdrive of Ipulse/Ic0−1. In principle, the switching is very sensitive to the initial condition θ0, which is distinct from the long pulse regime. Due to temperature fluctuations of the magnetization around the magnetization's equilibrium position, the divergent slowdown does not occur in such spin transfer switching. Thermal fluctuations cause the excursion of the magnetization from the easy axis by an average amount of √{square root over (kT/HKMsV)}, which generally aids the spin transfer based switching. However, thermal fluctuations may also cause a large distribution in switching time or current, which is undesirable for device applications.
In addition, depending upon the initial state of the magnetic storage element 38, the distribution in switching current across the MRAM 30 may change. Micromagnetic modeling indicates that the actual magnetic moment, for instance, of a standard elliptical magnetic storage element is not uniform. Instead, the magnetic moment has some distribution at the corners subject to the external driving force, such as an Oster field, due to existence of the writing current. For example, the moment distribution of particular magnetic storage elements 38 may include a C-shape state. In such a state there is no Oster field effect at the center portion of the magnetic storage element and the moment is aligned uniformly. However, at the corner the field effect is significant and tends to re-align the moment of the magnetic storage element 38 toward the C-state. This state thus stabilizes, or tends to lock, the moment from switching. Magnetic storage elements 38 in this state may require a higher current to switch, resulting in non-uniform switching. As a result, a wider distribution of current may exist. In addition, these two states may randomly occur in the switching process, depending upon factors such as previous switching processes and history of the magnetic storage element 38. It is, therefore, generally undesirable to commence a magnetization switching process with “C” state because of the potential for the formation of vortices during the switching. Additional energy is generally required to overcome a vortex for a full switching, giving rise to larger switching current with an increased distribution.
At higher densities, the conventional MRAM 30 may also suffer from size-related issues due to edge effects in the magnetic storage element 38. In order for the magnetic storage cell 31 of the conventional MRAM 30 to be written at small current, it is desirable to shrink the size of the magnetic storage cells 31. Size-related issues may occur for such smaller magnetic storage cells 31. As the size of the magnetic storage cells 31 decreases, the magnetic storage element 38 is also typically reduced in size. This reduction in size typically results in non-uniform and uncontrollable edge-fields for the magnetic storage element 38 that are produced by shape-anisotropy. The edge fields result in a large degree of randomly oriented magnetization vectors that form at the edge of the magnetic storage element 38. These magnetization vectors tend to curl back towards the magnetization vector of the body of the magnetic storage element 38. This tends to reduce the energy of the magnetic storage element 38. Such edge effects are also associated with uncompensated magnetic poles that form at the edge of the magnetic storage element 38. Moreover, as the size of the magnetic storage element 38 is decreased, the edge fields become at least as significant as the magnetization of the body of the magnetic storage element 38. Consequently, these edge effects may adversely affect on data write performance. Any irregularities of these shapes, defects at the edge produced during the formation, or uncompensated poles of variable strength, may result in coercivity fluctuation distributed throughout the MRAM 30 array. Thus, the edge effects and non-uniform array coercivity that become noticeable at smaller sizes adversely affect the conventional MRAM 30.
Consequently, although the MRAM 30 has a possibility of being used in higher density memories, issues related to initial conditions and the size of the magnetic storage element 38 persist. These issues adversely affect performance of the MRAM 30, for example resulting in a wider current distribution and a higher switching current. Accordingly, what is needed is a method and system that may improve performance of the current-based switching MRAM 30. The method and system addresses such a need.